1. Field of the Invention
The present invention relates to a method and to an apparatus for investigating the mechanical properties of tissue inside a live animal or human being. In particular, the present invention relates to the use of ultrasound for assessment of the mechanical properties of tissue. The invention more particularly relates to methods of investigating the mechanical properties of a material which may not be easily accessible and which may not be adequately described by simple theoretical models. The invention further relates to a method of investigating the mechanical properties of bone in vivo.
The investigation of the mechanical properties of bone in vivo is of great interest in view of the occurrence of osteoporosis. Osteoporosis may be defined as a long-term metabolic deficiency causing an unbalance in the natural process of bone resorbtion and bone rebuilding with the result of a loss of mechanical strength and increased risk of fractures. The degradation of the mechanical strength of the bones may proceed to a stage where even minimal trauma result in bone fractures. It has been estimated that osteoporosis affects from 10-20% or maybe even more of the female population and about 10% of the male population above the age of 50.
Treatments do exist which may delay or reverse the progression of osteoporosis. However, effective methods permitting an accurate diagnosis to be established at an early stage and permitting accurate monitoring of patient response to medication would be of great value.
Nevertheless, the accurate assessment of osteoporosis is difficult. The bones in the skeleton are by nature non-homogeneous and different parts of the skeleton may not be affected to the same degree. The material strength of the bones naturally changes over time, reaching a maximum about the age from 20-30 years and gradually declining later on. Individual differences may be substantial. Furthermore, the mechanisms involved and the factors controlling this process may not be fully understood.
2. The Prior Art
U.S. Pat. No. reissue Re32,782 discloses a method for determining in vivo strength of bone in a live being. According to this method, a first transducer launches an acoustic pulse though a bone and surrounding soft tissue, which pulse is received by a second transducer. The distance between the transducers is measured and so is the transit time of the pulse between the transducers. These measurements form the basis for determining the effective velocity of the pulse through the bone and surrounding soft tissue, which velocity provides one piece of information used for evaluating the strength of the bone.
The measurements of the distance and of the transit time in respect of an acoustic pulse launched through the bone with surrounding soft tissue provides no positive information about the path followed by the acoustic pulse. This may disturb the conclusions as the path may be non-linear due to refraction effects and as the acoustic transducers may exhibit strong sensitivity variations on varying orientations, both of which factors introduce potential sources of errors in case of accidental misalignments between the transducers. As the pulse is likely to have traveled through sections of soft tissue and through sections of bone tissue, any effect due to variations in bone structure may be buried in variations caused by other factors.
U.S. Pat. No. 5,038,787 discloses a method for analyzing material properties using reflected ultrasound. According to this method, a bone is placed in a separating medium that includes water and soft tissue and then subjected to ultrasound pulse waves from varying angles of incidence while the respective reflected waves are received and analyzed. The varying angles of incidence are obtained by shifting the transmitting transducer around. Critical angles of reflection are established and used in the calculation of a matrix of mechanical parameters such as elasticity.
The prior art also comprises a suggestion for providing a pair of transducers coaxially in a water tank, submerging a bone sample between the transducers, launching ultrasound pulses in various frequencies ranging from 0.2 MHz to 0.8 MHz and measuring the attenuation as a function of frequency.
In this method, any loss of signal will for the sake of the calculation be presumed as due to attenuation. This may amount to a misinterpretation, e.g. in case the loss was due to reflection, likely to arise whenever the pulse wave crosses some interface, e.g. water to soft tissue. In addition, this method does not separate contributions caused by effects in the soft tissue from contributions caused by effects in the bone tissue.
U.S. Pat. No. 4,408,492 refers in the context of ultrasonic echoscopy to the principle of time gain compensation, understood as the concept of increasing the gain of the receiver in proportion to the echo time delay in order to compensate for the progressive attenuation sustained by echoes from reflecting surfaces deeper within the object. The publication mentions the definition of a first and a second contour within the object subjected to the examination, the determination of the attenuation exhibited between the first and second contours and the use of the time gain compensation facility as necessary to compensate for the attenuation as determined. The purpose of this method is to overcome the problem of shadowing in a situation where a local highly absorbent area obscures deeper lying information.
U.S. Pat. No. 4,414,850 pertains to a measurement method and a system for measuring characteristics of attenuation of domains in an object, by which method ultrasonic waves are transmitted into the object and reflected ultrasonic waves are received and analyzed. Attenuation of the reflected waves is presumed to be a linear function of frequency. Attenuation coefficients are determined for various domains within the body.
U.S. Pat. No. 4,941,474 relates to a multi-variable analysis of bones for the purpose of detecting abnormal bone conditions. One method explained comprises launching into the patient an ultrasonic signal having components in the spectrum from about 100 kHz to about 600 kHz, receiving reflected signals, and evaluating the magnitude on the received signals.